Microtubules are cytoskeletal elements responsible for the spatial organization of cells. They carry out critical cellular processes such as chromosome segregation, cell polarization and cell migration. The dynamic behaviors of MT plus ends - their growth, shrinkage, targeting and attachment -- are modulated by a bevy of MT-associated factors. Here, we study the molecular mechanisms that regulate the MT cytoskeleton, using the fission yeast Schizosaccharomyces pombe as a model organism. Our studies focus on elucidating the mechanisms that regulate the transitions between MT growth and shrinkage, namely MT rescue and catastrophe. Our aims are: 1) to characterize CLASP, a candidate MT rescue factor that stabilizes MTs within the mitotic spindle and interphase MT bundles;2) to determine why MT catastrophe events occur at specific cortical sites at cell tips;3) to investigate possible roles of actin in the nucleus, focusing on its effect on chromosome segregation during mitosis. PUBLIC HEALTH RELEVANCE: Microtubules are dynamic filaments responsible for cell division and determination of cell shape and function. In this proposal we will determine how the growth and shrinkage of microtubules are regulated. These studies will provide greater understanding of how mitotic spindles work and will be relevant for human diseases such as cancer.